504 Minnesota Plant Life. 



snap together along the pulvinus-like midrib, quite after the 

 manner of a steel trap. The mere contact of an insect is 

 enough to set off the irritable mechanism. 



A tendril is a singularly ingenious contrivance. At first it 

 is straight, or with the free end slightly bent. It sweeps about 

 slowly with the growth-movement known as nutation, and w 7 hen 

 its tip encounters a resistant body, the tendril becomes bent 

 around the object that it has found and fastens itself to it. 

 Then the whole cylindrical organ coils into a spring, drawing 

 the leaf or stem up against the support. Later the tissues of 

 the tendril become woody, giving strength, while the coiled 

 structure gives elasticity. Thus plants with these organs are 

 the most perfect of climbers and support themselves lightly but 

 firmly upon their trellises. 



All the responses to stimulation that have just been reviewed 

 would be classified as curvatures. When a gall-forming insect 

 stings a plant, or when a witches'-broom fungus obtains a foot- 

 hold upon it, there is, rather, as a result of the stimulation, an 

 abnormal growth of tissue. Galls, in particular, are often very 

 extraordinary structures. The huge purple root-galls of the 

 wild rose, the spherical papery shells of the oak, the little hem- 

 ispherical nodules on basswoods leaves, the cone-like grey 

 bodies on willows and the great bushy tangles on black ashes 

 are all familiar objects in Minnesota. In such galls special 

 anatomical structures are called into existence by the virus in- 

 jected with the insect's sting. Special gall-types characterize 

 particular plants, and will not be found elsewhere. Between 

 the specific virus and the specific character of the living sub- 

 stance of the plant a relation exists ; and upon this depends the 

 anatomical and physiological character of the gall. 



Another type of induced irritability of organs is seen in the 

 arrangement of skeleton-tissues, for the large leaf will have more 

 strengthening material in the stem than will the small leaf of 

 the same species. Such strengthening areas are found to lie 

 very exactly in the regions of pressure or of tension, and may 

 be regarded as having come into being by the response of cells 

 to such stimuli. The cross-section of the familiar rhubarb 

 leaf-stalk shows it to be half-cylindrical in form. The arched 

 side is towards the ground and the calling into existence of 



